Hepatitis C virus (HCV) accounts for approximately 20% of cases of acute hepatitis, 70% of chronic hepatitis, and 30% of end-stage liver disease in the United States. The acute infection has an incubation period of 7 weeks (range, 4-20 weeks) and is symptomatic and icteric in only one third of patients. Serum aminotransferase levels generally increase greater than 10-fold elevated and as symptoms and signs resolve decrease into the normal range. Antibody to HCV is usually but not always present at the time of onset of symptoms. HCV RNA appears in the serum early during the incubation period, increases in titer and peaks at the time of symptoms, and then disappears in resolving disease. Importantly, 85% of patients with acute HCV infection develop chronic infection. In these patients, HCV RNA remains present and in approximately two thirds of patients, aminotransferases remain elevated in the range of 1.5- to 10-fold the upper limit of normal. The course of chronic hepatitis C is variable. Probably fewer than 20% of patients have symptoms and they are usually intermittent, vague, and nonspecific, largely being malaise and easy fatiguability. A small percentage of patients develop extrahepatic manifestations of hepatitis C, including cryoglobulinemia and glomerulonephritis. It is estimated that 20% to 30% of patients with chronic hepatitis C develop cirrhosis, but the process is generally slow and insidious. Once cirrhosis develops, symptoms are more common and the signs of end-stage liver disease can appear with jaundice, weakness, wasting, and gastrointestinal bleeding. Patients with cirrhosis are also at risk for developing hepatocellular carcinoma. Thus, this important liver disease has protean manifestations but is often insidious and can lead to end-stage liver disease despite the presence of few symptoms and signs of illness.